MOST ROBOTS USE electric power. The two main sources of untethered electric power for mobile robotics are batteries and photovoltaic cells. In a few years fuel cells will become a third electric power source for robotics.
Photovoltaic cells, commonly called solar cells, produce electric power from sunlight. A typical solar cell produces only a small amount of power, a few milliamperes at a potential difference of about 0.7 volts (V). Solar panels (modules) use many solar cells strung together to produce an appreciable power. The same is true with robotics. If enough solar cells are strung together, in series and parallel, sufficient power can be generated to operate a robot directly.
Solar-powered robots need to be designed as small as possible while still being able to perform their designated functions. They should be constructed using high-strength, lightweight materials and low-power electronics. The greater the weight reduction and the smaller the electric power consumption needed for operation and locomotion, the more viable the use of solar energy becomes. However, reduced weight and power consumption are important in the design of every robot. Lightweight, low-power robots are able to operate longer on a given power supply than their heavier, power-hungry counterparts. Solar cells can also indirectly power a robot by being used as a power source for recharging the robot's batteries. This hybrid power supply reduces the required capacity of the solar cells needed to operate the robot directly. However, the robot can only function for a percentage of the time that it spends recharging its power supply. We can also utilize solar cells by combining the technologies of direct and indirect power. Here we build what is commonly called a solar engine. The circuit is simple in function. The main components are a solar cell, main capacitor, and a triggering circuit. The solar cell when exposed to light begins charging a large capacitor. The solar cell/capacitor provides electric power to the rest of the circuit. As the charge on the capacitor increases, the voltage to the circuit also rises until it reaches a preset level that triggers the circuit. Once the cir- cuit is triggered, the power stored in the capacitor is dumped through the main load. The cycle then repeats. The solar engine may be used in a variety of innovative robotic designs.
Building a Solar Engine
The solar engine is commonly used as an onboard power plant for BEAM-type robots, sometimes called living robots . The inspiration for this solar engine originated from Mark Tilden, who originally designed a solar engine. Another innovator was Dave Hrynkiw from Canada, who modified the solar engine design to power a solar ball robot. In doing so i was able to create a new circuit that improved the efficiency of the original design. Figure 3.1 is the schematic for the solar engine. Here is how it works. The solar cell charges the main 4700-microfarad ( F) capacitor. As the capacitor charges, the voltage level of the circuit increases. The unijunction transistor (UJT) begins oscillating and sending a trigger pulse to the silicon controlled rectifier (SCR). When the circuit volt- age has risen to about 3 V from the main capacitor, the trigger pulse is sufficient to turn on the SCR. When the SCR turns on, all the stored power in the main capacitor is dumped through the high-efficiency (HE) motor. The motor spins momentarily as the capacitor discharges and then stops. The cycle repeats.
The solar engine circuit is simple and noncritical. It may be con- structed using point-to-point wiring on a prototyping breadboard. A printed circuit board (PCB) pattern is shown in Fig. 3.2 for those who want to make the PCB. The solar engine kit (see parts list) has a PCB included. Schematic for solar engine shows the PCB parts placement. The complete solar engine is shown in Schematic for solar engine.
Schematic for solar engine
Parts list for Solar Engine
- 2N2646 UJT transistor
- 2N5060 SCR
- 22- F cap
- 0.33-F cap
- DC motor
- Solar cells
- R1 200K ohm, 1 4 watt (W)
- R2 15K ohm, 1 4 W
- R3 2.2K ohm, 1 4 W
Not all electric motors are HE. For instance, the small electric motors sold at your local Radio Shack are of the low-efficiency type. There is a simple way to determine if a motor is an HE type. Spin the rotor of the motor. If it spins smoothly and continues to spin momentarily when it is released, it is probably an HE type.
PCB foil pattern
Parts placement on PCB
If when you spin the rotor it feels clunky or there is resistance, it probably is a low-efficiency type.
Caveats regarding the Solar Engine
The solar cell used in this circuit is high voltage, high efficiency. Typically solar cells supply approximately 0.5 to 0.7 V at various currents depending upon the size of the cell. The solar cell used in this circuit is rated at 2.5V, but it charge a capacitor up to 4.3V under no-load conditions.
You should not add a few more solar cells to speed charging. Adding solar cells will increase the current and will speed charging for the first cycle only. In order for the circuit to recycle, the current through the SCR must stop (or at least be very minimal) for the SCR to close. If there is too much current being supplied by the solar cell(s), the SCR will stay in the on condition. If this happens, the electric energy from the solar cell will continually flow through the SCR and be dissipated. Electric energy will not build on the capacitor, and the solar engine circuit will stop cycling. The components used in the circuit are balanced for proper operation. One component you may change is the main capacitor. You may use smaller values for quicker charge-and-discharge cycles.
A larger capacitor (or capacitor bank) will store more electric power and perform more work, but be aware that when using larger-value capacitors it will take that much longer to go through the charge-discharge cycle.
Complete solar engine
The attractiveness of the solar engine circuit is that it operates perpetually, or at least until one of the components breaks, which means it should operate for years.
In later articles we will study about:
- Battery power
- Battery voltage
- Primary batteries
- Secondary batteries
- Building a NiCd battery charger
- Building a solar-powered battery charger
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